Can you chain more than two devices

[vanDivX]

I have some problem registering Aastra 480i SIP phone as remote extension and I suspect LAN problem at that remote location.

The setup there is as follows: El cheapo DLink router modem with single LAN port which goes into IAX ATA box (S100FX) that has second pass through port to put PC behind it. Now I tried to put this Aastra phone (which also has pass through port) in beween the IAX box and the PC and it won't register at my PIAF although it is passing internet to the PC which is now third and last device in the chain.

I am working on getting the owner to phone up their ISP and get more uptodate router with four ports but in meantime I thought I would ask about the limit of chaining devices when you have more than the standard two on single physical port.

For now I assume I can't register the Aastra phone due to this setup, not because something is wrong at my remote end where PIAF resides.

 

[bmore]

It will not work reliably if there are multiple devices in the chain performing NAT on the packets. If there are multiple router/gateways in the chain, only one should be set to perform NAT. The others need to be in bridge mode or some devices will say routing mode v gateway mode. If not, you will get issues like failure to register, one way audio or no audio.

 

[jmullinix]

The Ethernet standard has, what we call, the 5-4-3 rule. Your lan can have 5 network segments, 4 repeaters (switches), and only 3 of those segments can have hosts on them.

When you start daisy chaining switches, which is what you have done, you risk running afoul of this rule.

A good network design is designed from the top down to avoid running into problems with this rule. In larger installations you will see a backbone switch, sometimes a fiber switch, that is connected to nothing but other switches. This second layer of switches go to the workstations. When you set things up this way, you never have more than 3 repeaters and 4 hops from any one host to any other host.

I think your network looks like this:

pbx ---- main switch ---- iax device--- Aastra phone --- PC

In this case you have 4 network segments and 3 hops, which is OK, but all 4 segments have hosts on them. This can cause weird problems.

 

[vanDivX]

@bmore: I didnt' think those phones are NATing, also doubt they have switch but most likely a simple hub to enable your mom and pop to hang PC behind those devices because they've got (like my sister at her house) single port router/modem connecting them to the world... but you are likely right.

@jmullinix - the topography is like this:

pbx ---[main switch] ---- internet ---- [main switch=modem/router NAT for LAN behind it] ----> iax device---> Aastra phone ---> PC

I didn't think of those pass-through boxes as switches, more like simple hubs as I said above but I suppose that makes no difference.

Since PBX is 'remote' it doesn't come into local network equation. In that case do I still have those 4 segments and 3 hops then as you said? Not sure how its counted and if you thought the PBX counts into it (or if the main switch counts too).
Also do I then have hosts on all 4 segments if I actually do have those four segments as you said?


It is sort of academic since I told the folks to call up their ISP and demand updated equipment and I am sure they will get something with minimum 4ports and hopefully WIFI. Problem is if you never call them up they leave you stuck with the router they gave you ages ago when dinosaurs were still roaming about

 

[jmullinix]

The 5-4-3 rule only applies to Lan's. The setup you have described does not run afoul of the rule.

Each one of the pass through devices is either a switch or a hub, but they still count as a hop. Make sure your IAX device is not working as a router and it should work.

 

[blanchae]

My understanding of the ports on a VoIP phone is one is connected to the Ethernet network and the other to a PC. Inside the phone is an Ethernet switch (not a hub). The Network port is to be connected to a multiVLAN port on a switch. This is a port that has two vlans assigned to it: voice and data.

In the phone configuration, there is a section where you can identify the voice vlan ID (number) and a priority (0-7 with 5 usually set for voice). The phone will use IEEE8021q vlan trunking to tag the frames with the vlan ID for the voice vlan. This tells the network switch which vlan the phone's traffic belongs to.

The voice traffic will have priority over the data traffic going through the phone's internal switch. Now this is all fine and dandy if you have a network vlan's but you don't. Because it is a switch, the 5-4-3 rule doesn't apply (only applies to hubs and repeaters).

Back to your original problem, I would first get the Astra phone registering by itself then the IAX ATA registering. Once that is established then try daisy chaining through the Astra phone then the IAX ATA.

 

[vanDivX]

Thank you for clearing up some of my confusion.
Up to now I didn't try to setup VLAN (I thought the V stands for Virtual LAN but perhaps that is what VoiceLAN is after all).
I do hope that the two places where I have those remote extensions will soon get multi port routers which will make things more simple and standard.

 

[Linetux]

John - a little late to the party, but I just need to add one comment about the "5-4-3" rule. Note this rule holds true on a segmented network connected with repeaters, or hubs (shared access). It does not hold true for switched environments, which almost everyone uses nowadays.

In reality, the Aastra is also a switch (has to be, as it does VLANs too).

 

[jmullinix]

I didn't know that. I have broken that rule a time or two in switched networks and they will fail. I think I had to get about 5 hops in before it failed though.

I also think it depends on how your switch is set up. Store-and-forward switches reset the hop count to 0, but pass-through (??? can't remember the official name) doesn't.